Axial turbine engines generally include fan, compressor, combustor and turbine sections positioned along an axial centerline sometimes referred to as the engine's “axis of rotation”. The fan, compressor, and combustor sections add work to air (also referred to as “core gas”) flowing through the engine. The turbine extracts work from the core gas to drive the fan and compressor sections. The fan, compressor, and turbine sections each include a series of stator and rotor assemblies. The stator assemblies, which do not rotate (but may have variable pitch vanes), increase the efficiency of the engine by guiding core gas flow into or out of the rotor assemblies.
Each rotor assembly typically includes a plurality of blades extending out from the circumference of a disk. Platforms extending laterally outward from each blade collectively form an inner radial flowpath boundary for core gas passing through the rotor assembly. An outer case, including blade outer air seals (BOAS), provides the outer radial flow path boundary. The blade outer air seal aligned with a particular rotor assembly is suspended in close proximity to the rotor blade tips to seal between the tips and the outer case. The sealing provided by the blade outer air seal helps to maintain core gas flow between rotor blades where the gas can be worked (or have work extracted).
Disparate thermal growth between the rotor assembly and the outer case can cause the rotor blade tips to “grow” radially and interfere with the aligned blade outer air seal. In some applications, the gap between the rotor blade tips and the blade outer air seal is increased to avoid the interference. A person of skill in the art will recognize, however, that increased gaps tend to detrimentally effect the performance of the engine, thereby limiting the value of this solution. In other applications, the blade outer air seals comprise an abradable material and the blade tips include an abrasive coating to encourage abrading of the blade outer air seals. The blade tips abrade the blade outer air seal until a customized clearance is left which minimizes leakage between the rotor blade tips and the blade outer air seal. In certain situations, unequal pressure loads may develop across the surface of the blade outer air seal, causing rotational forces that push the blade outer air seal out of the desired alignment.
Improvements are therefore needed in turbine engine rotor assembly blade outer air seals.